Wild-type type HAV, strain HM-175 RNA, was transcribed into cDNA and cloned. The cloned cDNA was used as a probe for detecting HAV RNA in tissue culture, serum, and fecal specimens by hybridization. Hybridization experiments also demonstrated that cDNA probes representing the HAV genome will not hybridize to RNA or cloned cDNA from a variety of other picornaviruses. From analysis of the nucleotide and predicted amino acid sequences of this genome, we have concluded that HAV has typical picornaviral genome organization but sequences that are quite different from other picornaviruses. We believe that it should be classified in a separate picornaviral genus. HAV VPg was demonstrated by immunoprecipitation. Encephalomyocarditis virus and vaccinia virus expression systems are being used in attempts to express the structural and nonstructural proteins of HAV. As an extension of hybridization studies with cloned HAV cDNA, RNA probes representing coxsackievirus sequences are being used in attempts to detect coxsackieviruses in infected tissue.